1. Field of the Invention
The present invention relates to an electrostatic latent image forming apparatus. More specifically, the present invention relates to an electrostatic latent image forming apparatus in which an electrostatic latent image is formed on a dielectric film by partially applying electric charges onto the same or by partially removing electric charges from a dielectric film being uniformly charged in advance.
2. Description of the Prior Art
As this kind of an electrostatic latent image forming apparatus, typically, an apparatus using a multi-styluses head is known. In one example of such an apparatus using the multi-styluses head, the dielectric drum is uniformly charged by a charging corotron, and a pulse voltage having a reverse polarity of the charged polarity is applied to the multi-styluses head which is held at a distance of 10 .mu.m through 20 .mu.m from a surface of the dielectric drum to make desired styluses take place discharge. By partially removing or reducing the electric charges being charged on the dielectric drum by means of the discharge of the styluses, an electrostatic latent image is formed on the dielectric drum.
In the apparatus using such a multi-styluses head, it is difficult to minimize the size of the apparatus and to lower manufacturing cost thereof. Furthermore, there is a problem that it needs MOSFET having a high withstand voltage to apply a high voltage pulse to the multi-styluses head.
By contrast, an electrostatic latent image forming apparatus not using the multi-styluses head is disclosed in, for example, Japanese Patent Laying-Open No. 58-207058 laid open on Dec. 2, 1983. In this secondary prior art, a photoconductive layer is formed on a glass base plate via a transparent electrode, and a light switch array which is arranged closely to a surface of a dielectric drum and a light switch driving means for applying a light signal in accordance with an image signal to the light switch array are provided. By making discharge between the dielectric drum and the light switch array, an electrostatic latent image in accordance with the image signal is formed on the surface of the dielectric drum. The secondary prior art is effective to minimize the size of the apparatus or to reduce the cost since the same does not use the above-described multi-styluses head, but the following problem cannot be solved.
In the case where a photoconductive material is caused to make a switching operation by impinging a light thereto so as to give a photo-conductivity to the same, in accordance with a general characteristic of a light, the light is strongly absorbed at a surface side portion of the photoconductive layer where the light is impinged, and the amount of light is decreased at a portion of the dielectric drum side where discharge takes place, and therefore the photo-conductivity of that portion is decreased. Accordingly, in the photoconductive layer, a resistance is gradually increased from the light impinged side to the dielectric drum side. In a portion where the resistance of the photoconductive layer is high, electric charges able to move a direction in parallel with the surface of the photoconductive layer, therefore an area of a spot-like light signal is gradually expanded to the dielectric drum side in the photoconductive layer, and therefore unevenness of density of the electric charges being discharged takes place. Accordingly, quality of the electrostatic latent image as formed, especially resolution thereof is decreased.